Solder paste is widely used in the electronic industry. For example, solder paste reflow is used for surface mount assembly of packaged electronic components onto circuit boards, and to form solder balls (bumps) on ball grid array (BGA) electronic packages. For surface mounting applications which use solder paste/reflow technology, interconnections to an electronic component such as a packaged IC unit (integrated circuit chip or multi-chip module) or a discrete component (a resistor, a capacitor, a transistor or a diode) take the form of leaded or unleaded arrays of solderable I/O (input-output) connections. Examples include surface mountable IC packages with perimeter arrays of solder wettable butt, J or gull-wing shaped metal leads, surface mountable discrete components with solder wettable contact pads and surface mountable BGA packages with area or grid arrays of solder wettable OF pads, each with its own solder ball. The "surface mount" assembly process involves placing individual deposits of solder paste over each of the solder wettable contact pads of the main circuit board and aligning the array of I/O contacts (IC leads, discrete component contact pads or BGA solder balls) on the component to the pattern of solder paste deposits that was placed on the contact pads of the main circuit board. The solder wettable I/O contacts of the surface mountable package unit are then placed into the solder paste deposits. The process of alignment and placement is repeated until the main circuit board is fully populated. This assembly is then heated so as to melt the solder in the paste such that, upon subsequent cooling and solidification, the solder forms joints. As a result of forming these solder joints, all of the I/O contacts on every component are bonded electrically and mechanically to the appropriate contact pads on the main circuit board. This process is know as a solder paste "reflow" assembly.
Another application of solder paste is to form solder balls on an area array of solder wettable metal pads that comprise the I/O contacts on a surface mountable ball grid array (BGA) package. Such an application is described by Y. Degani, T. D. Dudderar and W. L. Woods, Jr. in U.S. application Ser. No. 08/128492 filed Sep. 23, 1993. This patent application discloses an expedient process for producing solder balls (or bumps) on a grid array of solder wettable metal I/O pads. In this process solder paste is stencil printed onto the solder wettable metal I/O pads on the bottom side of the BGA and then reflowed to form solder balls (or bumps) on each pad. This process is known as printed solder "bumping", and the resulting solder bumps or balls comprise the I/O contacts for applications such as surface mount reflow assembly of the BGA packages onto printed circuit boards as described above.
In both of these applications the solder paste is comprised of a flux system plus a solder alloy in powder form. In order to assure that the resulting solder joints be consistently uniform and reliable, it is important that the paste be designed so that most of the solder powder will consolidate into a single mass during the reflow process. Here "most of the solder" means at least 97%, preferably 99%, most preferably 99.5% (by weight) of the total solder powder. Furthermore, for surface mount assembly it is also necessary that this single, well consolidated mass of reflowed solder be formed on and wetted to the solder wettable metal pad on which the paste was initially printed and robustly wetted to the packaged IC lead, discrete component contact pad or BGA solder ball. Solder pastes which incorporate solder powders of eutectic or near-eutectic alloys, such as 63/37 Sn/Pb or 60/40 Sn/Pb, respectively, will occasionally fail to meet these criteria. For example, (1) in the surface mount assembly of components to a circuit board, occasionally a stray non-consolidated (eutectic or near-eutectic alloy) solder ball will be found under the component after reflow, or (2) in bumping a BGA with a eutectic or near eutectic alloy solder paste as described above, occasionally a well-consolidated solder ball will be found in a location well off of the solder wettable metal I/O pad after reflow. Since such a misplaced solder ball is not in contact with the circuitry, the entire BGA package is rendered unusable. On the other hand, in the case of example (1), the stray solder ball is a potential reliability risk in terms of either an insufficient solder joint volume or an electrical short. (See H. H. Manko, Soldering Handbook for Printed Circuits and Surface Mounting, pages 290 to 294, Van Nestrand Reinhold (New York ) 1986.
Clearly, a need exists for solder paste compositions which, when applied to surface mount assembly or BGA solder ball formation, result in an uncompromised consolidation and wetting process in which all of the solder joints or balls are uniform, and every uniform solder .joint or ball is formed on and wetted to an appropriate contact by the solder reflow process.